1. Field of the Invention
The present invention relates to a method for producing an alcohol. More specifically, it relates to a method for producing an alcohol comprising continuously supplying and flowing starting materials of an alcohol in the presence of a hydrogenation catalyst and carrying out catalytic reduction reaction of the starting materials to give an alcohol.
2. Discussion of the Related Art
Methods for producing alcohols comprising carrying out reduction reaction of starting materials of an alcohol, such as a fatty acid ester, a fatty acid triglyceride, and a fatty acid, in the presence of a hydrogenation catalyst have been generally carried out under conditions of a pressure of from 200 to 300 atm. and a temperature of from 200.degree. to 300.degree. C. In view of carrying out such methods in an actual production plant, however, a process at such a high pressure necessitates not only equipments with sufficient pressure resistance but also sufficient maintenance of the equipments for keeping air-tightness. Therefore, such equipments would require both of high plant investments and high running costs.
In order to overcome these problems and produce more inexpensive alcohols, various studies have been recently made on the methods for hydrogenation reaction at a relatively low pressure. The methods are disclosed, for instance, in U.S. Pat. No. 5,475,159, Japanese Patent Unexamined Publication No. Hei 4-504408, and Japanese Patent Laid-Open No. Hei 7-188077.
U.S. Pat. No. 5,475,159 discloses a method for producing an aliphatic alcohol comprising hydrogenating a methyl ester of a fatty acid in a liquid-phase reaction under the conditions of a reaction pressure of from 300 to 2000 psig and a reaction temperature of from 170.degree. to 250.degree. C.
Under the above-mentioned conditions, however, a reaction intermediate, a wax ester, is included in a large amount at the outlet of the reactor, and hence, the yield of alcohol is liable to be lowered.
The reasons for a low yield of alcohol are presumably as follows: In the hydrogenation reaction, the amount of hydrogen dissolved in an ester constituting a liquid phase in the reaction system is decreased as the reaction pressure is decreased, thereby drastically lowering the reaction activity. In addition, the chemical reaction during production of an alcohol is an equilibrium reaction, wherein the reaction equilibrium is shifted towards the side where the reaction intermediate, the wax ester, remains in the resulting alcohol under the given pressure conditions mentioned above.
In addition, there are advantages in that since the wax ester has a different boiling point from the formed alcohol and thus can be easily separated by distillation, the lowering in the yield of the alcohol can be prevented, and that the wax ester can be employed again in the hydrogenation process as a starting material for an alcohol after separation and collection of the wax ester. There is, however, a defect in necessitating a plant investment for the separation and collection processes.
On the other hand, Japanese Patent Unexamined Publication No. Hei 4-504408 discloses a method for producing an aliphatic alcohol comprising hydrogenating a lower alkyl ester of a fatty acid in a vapor-phase reaction under the conditions of a reaction pressure of from 5 to 100 bar, a reaction temperature of from 140.degree. to 240.degree. C., and a molar ratio of hydrogen to ester of from 200:1 to 2000:1, wherein a mixture of the ester and hydrogen gas brought into contact with a hydrogenation catalyst is constantly at a temperature higher than its dew point. In addition, Japanese Patent Laid-Open No. Hei 7-188077 discloses a method for producing alcohols comprising the step of hydrogenating esters in a vapor-phase reaction under the conditions of a reaction pressure of from 150 to 2000 psia, a reaction temperature of from 150 to 350.degree. C., and a molar ratio of hydrogen of from 10 to 8000, wherein the temperature conditions are such that the starting material stream is fed at a temperature higher than its dew point by not less than 5.degree. C.
By carrying out the hydrogenation reaction in the vapor phase under the above-mentioned conditions, the conventional problem regarding the amount of hydrogen gas dissolved in the liquid phase under low pressure conditions can be solved, and thereby the reaction activity is remarkably improved. Moreover, since the reaction equilibrium during the production of alcohols is largely shifted toward the side of formation of alcohols in the vapor phase, the amount of the wax ester included in the resulting alcohols at the outlet of the reactor can be expected to be decreased.
Actually, however, a side reaction, namely, the reduction reaction of alcohols to hydrocarbons, is also accelerated, which results in poor selectivity though having excellent reactivity, and hence, the yield is rather notably decreased.
Additionally, in the case where an ester having 8 to 18 carbon atoms is hydrogenated under the above-mentioned conditions, the boiling point region of a hydrocarbon formed as a side-product overlaps with that of a short chain aliphatic alcohol, and hence, it is made difficult to separate the hydrocarbons from the alcohol in the subsequent processes by distillation. Therefore, in order to carry out hydrogenation reaction under the above-mentioned conditions, separation of a short chain fraction from a long chain fraction in the ester prior to the hydrogenation reaction is necessitated.
Accordingly, one object of the present invention is to provide a method for producing an alcohol comprising carrying out catalytic reduction reaction of starting materials of an alcohol in the presence of a hydrogenation catalyst, wherein the method can be carried out without necessitating a step of separating the starting materials depending upon the chain length prior to the reduction reaction, and having excellent reactivity and selectivity even at a low reaction pressure; and specifically, to provide a method for producing an alcohol wherein the amounts of the starting materials of an alcohol remaining unreacted, the reaction intermediate, the wax ester, and the side-products, the hydrocarbons, included in the resulting alcohol are lowered to such an extent that a purification process such as distillation is not necessitated, and thereby making it possible to improve the yield of the alcohol.
These and other objects of the present invention will be apparent from the following description.